Integrally-geared compressors for precooling in lng applications

ABSTRACT

A natural gas liquefaction system is disclosed, which comprises at least a pre-cooling loop, through which a first refrigerant is adapted to circulate. The pre-cooling loop comprises at least one compressor for pressurizing the first refrigerant; at least one prime mover for driving the compressor; at least one condenser for removing heat from the first refrigerant; at least a first expansion element for expanding the first refrigerant; at least a first heat exchanger for transferring heat from natural gas to the first refrigerant. The system further comprises at least a cooling loop, downstream of the pre-cooling loop, where through a second refrigerant circulates. The natural gas is adapted to be sequentially cooled in the pre-cooling loop and in the cooling loop. The compressor of the pre-cooling loop is an integrally-geared turbo-compressor comprising a plurality of compressor stages.

BACKGROUND

1. Field of the Invention

The embodiments disclosed herein relate to processes and systems for liquefying natural gas.

2. Description of the Related Art

Natural gas is becoming an increasingly important source of energy. In order to allow transportation of the natural gas from the source of supply to the place of use, the volume of the gas must be reduced. Cryogenic liquefaction has become a routinely practiced process for converting the natural gas into a liquid, which is more convenient, less expensive and safer to store and transport. Transportation by pipeline or ship vessels of liquefied natural gas (LNG) becomes possible at ambient pressure, by keeping the chilled and liquefied gas at a temperature lower than liquefaction temperature at ambient pressure.

In order to store and transport natural gas in the liquid state, the natural gas in an embodiment is cooled at around −150 to −170° C., where the gas possesses a nearly atmospheric vapor pressure.

Several processes and systems exist in the prior art for the liquefaction of natural gas, which provide for sequentially passing the natural gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled in successively lower temperatures in sequential refrigeration cycles until the liquefaction temperature is achieved.

Prior to passing the natural gas through the cooling stages, the natural gas is pre-treated to remove any impurities that can interfere the processing, damage the machinery or are undesired in the final product. Impurities include acid gases, sulfur compounds, carbon dioxide, mercaptans, water and mercury. The pre-treated gas from which impurities have been removed is then cooled by refrigerant streams to separate heavier hydrocarbons. The remaining gas mainly consists of methane and usually contains less than 0.1% mol of hydrocarbons of higher molecular weight, such as propane or heavier hydrocarbons. The thus cleaned and purified natural gas is cooled down to the final temperature in a cryogenic section. The resulting LNG can be stored and transported at nearly atmospheric pressure.

Cryogenic liquefaction is usually performed by means of a multi-cycle process, i.e. a process using different refrigeration cycles. Depending upon the kind of process, each cycle can use a different refrigerating fluid, or else the same refrigerating fluid can be used in two or more cycles.

FIG. 1 schematically shows a diagram of a cryogenic natural gas liquefaction system using the so-called APCI process. This known process uses two refrigeration cycles. A first cycle uses propane as a refrigeration fluid and a second cycle uses a mixed refrigerant, usually made of nitrogen, methane, ethane and propane. The system, labeled 1 as a whole, comprises a first cycle 2 comprising a line formed by a gas turbine 3, which drives a compressor train. The compressor train comprises a first compressor 5 and a second compressor 7 in series for compressing the mixed refrigerant. An inter-stage cooler (inter-cooler) 9 cools the mixed refrigerant delivered by the first compressor 5 to reduce the temperature and the volume thereof before entering the second compressor 7. The compressed mixed refrigerant delivered by the second compressor 7 is condensed against air or water in a heat exchanger 11. The mixed refrigerant is further cooled and partly liquefied by the propane cycle 12 as disclosed here below.

The propane is processed in a second or pre-cooling cycle. The second cycle comprises a line including a gas turbine 13, which drives a multi-stage compressor 15. The compressed propane delivered by compressor 15 is condensed in a condenser 17 against water or air. The condensed propane is used to pre-cool the natural gas down to −40° C. and to cool and partially liquefy the mixed refrigerant. The natural gas pre-cooling and the mixed refrigerant partial liquefaction is performed in a multi-pressure process, in the example shown 4-level of pressure.

The stream of condensed propane from condenser 17 is delivered to a first set of four, serially arranged heat exchangers to cool and partly liquefy the mixed refrigerant and to a second set of four, serially arranged, pre-cooling heat exchangers to cool the natural gas. A first portion of the compressed propane stream from condenser 17 is delivered through pipe 19 to the first set of heat exchangers and is sequentially expanded in serially arranged expanders 21, 23, 25 and 27 to four different, gradually decreasing pressure levels. Downstream each expander 21, 23 and 25 a portion of the expanded propane flow is diverted to a respective heat exchanger 29, 31, 33. The propane flowing through the last expander 27 is delivered to a heat exchanger 35.

The compressed mixed refrigerant delivered from the heat exchanger 11 flows in a pipe 37 towards a main cryogenic heat exchanger 38. The pipe 37 sequentially passes through the heat exchangers 29, 31, 33 and 35, such that the mixed refrigerant is gradually cooled and partly liquefied against the expanded propane.

A second fraction of the condensed propane flow from condenser 17 is delivered to a second pipe 39 and expanded sequentially in four serially arranged expanders 41, 43, 45 and 47. A part of the propane expanded in each expander 41, 43 and 45 as well as the propane flowing from the last expander 47 is diverted towards a corresponding pre-cooling heat exchanger 49, 51, 53 and 55, respectively. A main natural gas line 61 flows sequentially through the pre-cooling heat exchangers 49, 51, 53 and 55, such that the natural gas is pre-cooled before entering the main cryogenic heat exchanger 38. Heated propane exiting the pre-cooling heat exchangers 49, 51, 53 and 55 is collected with the propane exiting the heat exchangers 29, 31, 33 and 35 and is fed again to the compressor 15, which recovers the four evaporated propane side streams and compresses the vapor to e.g. 15-25 bar to be condensed again in condenser 17.

SUMMARY OF THE INVENTION

The subject matter disclosed herein concerns an improved natural gas liquefaction system comprising at least a pre-cooling circuit or loop wherein a first refrigerant is caused to circulate, and at least one cooling or liquefying loop, wherein a second refrigerant is caused to circulate. A natural gas in the gaseous state is caused to flow through a heat exchangers arrangement of the pre-cooling loop and subsequently in a heat exchangers arrangement of the cooling or liquefying loop. The natural gas is pre-cooled, cooled and finally liquefied by exchanging heat against the first refrigerant and at least the second refrigerant. Additional third or further cooling and/or liquefying circuits or loops can be arranged in a cascade or sequence arrangement to gradually chill and finally liquefy the natural gas. The loops contain respective compressor arrangements for processing the respective refrigerants, as well as at least one condenser and one or more expansion elements, e.g. turboexpanders and/or throttling valves. At least the pre-cooling loop comprises an integrally-geared turbo-compressor for processing the first refrigerant. The first refrigerant can be divided into two or more side streams, used to exchange heat at gradually decreasing pressure values, against the natural gas and/or the refrigerant circulating in the subsequent cooling or liquefying loop.

According to some embodiments, a natural gas liquefaction system is provided, comprising: at least a pre-cooling loop, through which a first refrigerant is adapted to circulate, the pre-cooling loop comprising: at least one compressor for pressurizing the first refrigerant; at least one prime mover for driving the compressor; at least one condenser for removing heat from the first refrigerant; at least a first expansion element for expanding the first refrigerant; at least a first heat exchanger for transferring heat from natural gas to the first refrigerant; and at least a cooling loop, downstream of the pre-cooling loop, where through a second refrigerant circulates, the natural gas being adapted to be sequentially cooled in the pre-cooling loop and in the cooling loop; wherein the compressor is an integrally-geared turbo-compressor comprising a plurality of compressor stages each one being provided with an independent set of movable inlet guide vanes for autonomously regulating flows entering in the compressor stages.

According to some embodiments, additional compressor stages can be provided, which are not provided with movable inlet guide vanes. When a plurality of compressor stages are disposed in series, a single set of movable inlet vanes is enough, since the downstream stages are regulated by the set of movable inlet vanes of the more upstream stage. Compressor stages disposed in series could be equipped with respective set of movable inlet vanes when the first refrigerant stream is divided in two or more side streams and successively re-united in an intermediate position between two subsequent compressor stages.

According to another aspect, a method of liquefying natural gas, is provided, wherein a flow of natural gas is cooled and liquefied by heat exchange against at least a first refrigerant circulating in a pre-cooling loop and a second refrigerant circulating in a cooling and/or liquefying loop. The first refrigerant is divided into a plurality of side streams at gradually decreasing pressure values. The side streams exchange heat against the natural gas flow and/or against the second refrigerant. The side streams are returned at respective compressor stages of an integrally-geared turbo-compressor.

According to one embodiment, a method is provided, comprising: providing a pre-cooling loop comprising: an integrally-geared turbo-compressor having a plurality of compressor stages, at least one condenser, at least one expansion element, and at least one heat exchanger; driving the integrally-geared turbo-compressor with a prime mover; circulating a first refrigerant through the integrally-geared turbo-compressor; condensing the first refrigerant delivered by the integrally-geared turbo-compressor in the condenser; dividing the first refrigerant in a plurality of partial flows; expanding the condensed first refrigerant in the expansion element; circulating the expanded refrigerant through the heat exchanger to remove heat from the natural gas, to pre-cool the natural gas; controlling independently movable inlet guide vanes to regulate the partial flows at the suction side of the compressor stages; providing at least one cooling loop; circulating a second refrigerant in the at least one cooling loop; remove heat from the pre-cooled natural gas by heat exchange against the second refrigerant.

Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

The flow rates of first refrigerant elaborated by the present system and method are controllable acting not only to the rotational speed of compressor stages. In this way, a more efficient and reliable LNG circuit is provided.

As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates a system for liquefying natural gas according to the current art;

FIG. 2 illustrates a schematic of a first embodiment of a system for the production of LNG according to the present disclosure;

FIG. 2A illustrates an exemplary embodiment of an integrally geared compressor used in the arrangement of FIG. 2;

FIG. 3 illustrates a schematic of a system for the production of LNG according to the present disclosure, in a second embodiment; and

FIG. 4 illustrates section of two compressor stages of an integrally-geared turbo-compressor for use in a LNG system according to the present disclosure.

DETAILED DESCRIPTION

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

FIG. 2 schematically shows a diagram of a cryogenic natural gas liquefaction system based on the APCI process embodying the subject matter disclosed herein. The process uses two refrigeration cycles or loops, wherein a first refrigerant and a second refrigerant, respectively, are processed. The first loop is a pre-cooling loop wherein the natural gas as well as the second refrigerant, circulating in a second loop, are cooled by exchanging heat with the first refrigerant. Herein after the first loop will be referred to as pre-cooling loop or cycle and the second loop will be referred to as the cooling or liquefaction loop or cycle.

In some embodiments the first refrigerant circulating in the pre-cooling loop can include or consist of propane. The first refrigerant can have a mean molecular weight of at least 35, for example between 35 and 41. In some embodiments the second refrigerant circulating in the second loop can include a mixed refrigerant, for example comprising nitrogen, methane, ethane and propane.

More specifically, in the embodiment of FIG. 2 the system is labeled 101 as a whole, the first or pre-cooling loop is labeled 103 and the second liquefaction cycle or loop is labeled 105. Natural gas is delivered to the system 101 along a pipe 107 and is sequentially cooled and finally liquefied by flowing through a plurality of serially arranged heat exchangers of the pre-cooling loop 103 and of the cooling loop 105, respectively.

The pre-cooling loop 103 comprises a multi-stage, integrally-geared turbo-compressor 109. The integrally-geared turbo-compressor can be configured as shown in more detail in FIG. 4, and will be described in greater detail later on, reference being made to the figure.

At least one, some, or more particularly all the stages of the integrally-geared turbo-compressor are comprised of movable inlet guide vanes, to adjust the operative conditions of the stage(s) according to the actual operative needs of the system 101. Each set of movable inlet guide vanes can be adjusted independently of the other, for instance in order to take into account flow rates which differ from one stage to the other.

In some embodiments the integrally-geared turbo-compressor comprises a number of stages comprised between two and eight. For example, the integrally-geared turbo-compressor can comprise from three to six stages. As will be described in more detail later on, one or more inter-coolers can be provided between one or more pairs of sequentially arranged stages of the integrally-geared turbo-compressor. Moreover, in some embodiments, at least one, some, or preferably all the stages of the integrally-geared turbo-compressor are comprised of movable inlet guide vanes, to adjust the operative conditions of said stage(s) according to the actual operative needs of the system 101. Each set of movable inlet guide vanes can be adjusted independently of the other, for instance in order to take into account flow rates which differ from one stage to the other.

In some embodiments the multi-stage, integrally-geared turbo-compressor 109 can be driven by a prime mover, which can include an internal combustion motor, such as a gas turbine, for instance an aeroderivative gas turbine. In some embodiments, the integrally-geared turbo-compressor 109 is driven by an electric motor 111.

In FIG. 2 an exemplary embodiment is shown, wherein the integrally-geared turbo-compressor 109 is comprised of four stages, labeled 109A, 109B, 109C, 109D, respectively, arranged in sequence, stage 109D being the lowest-pressure stage, and stage 109A being the highest-pressure stage.

A flow of compressed first refrigerant is delivered by the integrally-geared turbo-compressor 109 to a condenser 115. The flow of first refrigerant delivered through the condenser 115 is cooled, e.g. against water or air, and condensed.

In some embodiments, the condensed first refrigerant is circulated in the pre-cooling loop 103 to pre-cool the natural gas and to cool and optionally partially liquefy the second refrigerant circulating in the cooling loop 105.

In some embodiments, the process is divided into four pressure levels. The number of pressure levels can correspond to the number of stages of the integrally-geared turbo-compressor 109. In some embodiments, the flow of first refrigerant delivered through the condenser 115 is divided into a number of partial flows, which are then sequentially expanded at a number of progressively reducing pressure levels. Each partial refrigerant flow circulates in a sub-cycle and is returned as a side flow to the integrally-geared turbo-compressor at the inlet of a corresponding one of the plurality of compressor stages.

A delivery line 117 delivers a first part of the condensed first refrigerant flow to a plurality of serially arranged first expansion elements 119A-119D. A second delivery line 118 branched-off the delivery line 117 delivers a second part of the condensed first refrigerant flow to a plurality of serially arranged second expansion elements 121A-121D.

The first part of the condensed first refrigerant from condenser 115 is sequentially expanded in the four expansion elements 119A-119B at four different, gradually decreasing pressure levels. Downstream each expansion element 119A-119C a portion of the flow of partly expanded first refrigerant is diverted to a respective one of first, pre-cooling heat exchangers 123A-123C. The remaining part of the partly expanded first refrigerant is caused to flow through the next expansion element 119A-119C and so on. The residual first refrigerant flowing through the most downstream one (119D) of the first expansion elements 119A-119D is delivered to a most downstream pre-cooling heat exchanger 123D.

In each one of the first heat exchangers 123A-123D the first refrigerant exchanges heat against the natural gas flowing in pipe 107, thus pre-cooling and optionally partly liquefying the natural gas.

A part of the first refrigerant expanded in each second expansion elements 121A, 121B, 121C is diverted towards a corresponding one of a plurality of second heat exchangers 125A-125D. The part of refrigerant flow delivered by each one of the second expansion elements 121A-121C and which is not caused to flow through the respective heat exchanger 125A-125C is delivered through the subsequent expansion element. The most downstream one (125D) of the second heat exchangers receives the entire residual fraction of first refrigerant expanding in the most downstream (121D) of the second expansion elements 121A-121D. In each one of the second heat exchangers 125A-125D the first refrigerant exchanges heat against the second refrigerant, circulating in the cooling or liquefying loop 105, so that at the delivery side of the heat exchanger 125D the second refrigerant is cooled and at least partly liquefied.

Heated first refrigerant exiting the first, pre-cooling heat exchangers 123A-123D is collected with the heated first refrigerant exiting the second heat exchangers 125A-125D and is fed again to the integrally-geared turbo-compressor 109.

In some embodiments the heated first refrigerant exiting each second heat exchanger 125A-125D is at around the same pressure as the heated first refrigerant exiting the corresponding first heat exchanger 123A-123D. The refrigerant collected at corresponding pressure levels is delivered at the inlet of corresponding stages of the integrally-geared turbo-compressor 109. A plurality of refrigerant side streams are thus returned at gradually decreasing pressures at the inlet of the serially arranged stages of the integrally-geared turbo-compressor 109.

In FIG. 2 reference numbers 130A-130D indicate return lines, through which the side streams of expanded and exhausted refrigerant fluid delivered from the heat exchangers 123A-123D and 125A-125D are returned to corresponding stages 109A-109D of the integrally-geared turbo-compressor.

In some embodiments, the cooling or liquefying loop 105 comprises a compressor train. In some embodiments the compressor train can be comprised of a first compressor 131 and a second compressor 133 arranged in series. In other embodiments a single compressor can be provided. Each compressor can be a multi-stage compressor, for example a multi-stage centrifugal compressor.

In some embodiments the compressor(s) of the cooling loop 105 are driven by a prime mover, which can include an internal combustion engine. The prime mover can be a gas turbine 135, for instance an aeroderivative gas turbine.

An inter-stage cooler (inter-cooler) 137 can be arranged between the first compressor 131 and the second compressor 133, to reduce the temperature and the volume of the second refrigerant delivered by the first compressor 131 before entering the second compressor 133. The compressed second refrigerant delivered by the second compressor 133 is condensed in a condenser 139. The condenser 139 can be an air condenser or a water condenser, where the second refrigerant is condensed by exchanging heat against air or water. The condensed second refrigerant is subsequently delivered by a delivery line 141 through the sequentially arranged second heat exchangers 125A-125D, where the second refrigerant is cooled and possibly liquefied by exchanging heat against the first refrigerant circulating in the pre-cooling loop 103, as described above.

The cooled, and optionally partly liquefied second refrigerant delivered from the heat exchangers 125A-125D flows through a pipe 143 towards a main cryogenic heat exchanger 145, where the second refrigerant removes further heat from the pre-cooled and optionally partly liquefied natural gas, completing the liquefaction process. The entirely liquefied natural gas exits the system at 149, and the heated second refrigerant is returned through a line 151 to the compressor(s) or compressor train 131, 133.

In FIG. 2 the integrally-geared turbo-compressor 109 is represented only schematically. The main components of an exemplary integrally-geared turbo-compressor 109 are illustrated in more detail in FIG. 2A. FIG. 4 illustrates in more detail an axial section of two compressor stages supported on a common rotary shaft of the integrally-geared turbo-compressor 109. More specifically, FIG. 4 illustrates by way of example the first and second stages 109D, 109C.

More particularly, each compressor stage 109A-109D is provided with movable inlet guide vanes, schematically shown at 110A-110D for the four stages 109A-109D. In other embodiments, movable inlet guide vanes are provided at the inlet of only some or none of the compressor stages. As can be appreciated from FIG. 4, the inlet guide vanes can be arranged at the axial inlet of the compressor stage. Each set of movable inlet guide vanes can be controlled independently of the other sets for autonomously regulating flows entering in the compressor stages. An intercooler can be provided between two sequentially arranged compressor stages 109A-109D. As shown in FIG. 2A, a first intercooler 153 can be arranged between the delivery side of the first compressor stage 109D and the suction side of the second compressor stage 109C. A second intercooler 155 can be arranged between the delivery side of the second compressor stage 109C and the suction side of the third compressor stage 109B. A third intercooler 157 can be arranged between the delivery side of the third compressor stage 109B and the suction side of the fourth compressor stage 109A.

Each compressor stage 109A-109D comprises at least one impeller supported on a rotary shaft. FIG. 4 shows two impellers 112D, 112C of the two most upstream compressor stages 109D, 109C, respectively. Each impeller can be a radial impeller, with an axial inlet and a radial outlet. The fluid processed through the impeller is collected in a respective volute, such as volutes 114D, 114C of compressor stages 109D, 109C.

The impellers can be paired, each pair of impellers being supported by a common rotary shaft. In the embodiment of FIG. 2A two rotary shafts 159, 161 are provided. The impellers of the first and second compressor stages 109D, 109C are mounted for rotation on the first rotary shaft 159 and the impellers of the third and fourth compressor stages 109B, 109A are mounted for rotation on the second rotary shaft 161. A different number of rotary shafts and respective compressor stages and impellers can be provided. In some embodiments an odd number of stages can be provided, in which case one of the rotary shafts can support a single impeller instead of paired impellers.

Each rotary shaft 159, 161 comprise a pinion 159A, 161A keyed thereon. The pinions 159A, 161A mesh with a central toothed wheel or crown 163 which is driven in rotation by the electric motor 111 through a driving shaft 165. The two rotary shafts 159A, 161A and therefore the respective impellers mounted thereon can rotate at different rotary speeds.

The structure of the integrally-geared turbo-compressor 109 is particularly suitable for processing the different side streams of the first refrigerant circulating in the pre-cooling loop 103. The position of each set of movable inlet guide vanes 110A-110D at the inlet of the compressor stages can be adapted to the flow conditions of each side stream, i.e. each refrigerant stream delivered to the respective suction side of the compressor stages, so that the operative conditions of the compressor stages can be adapted to the temperature conditions and flow rates through the different heat exchangers 123A-123D, 125A-125D. The compressor efficiency and operability can thus be maximized. One or more intercoolers, such as intercoolers 153, 155, 157 easily integrated in the structure of the integrally-geared turbo-compressor 109 further increase the efficiency of the compressor and thus of the whole LNG system.

A further embodiment of the subject matter disclosed herein is illustrated in FIG. 3 and will be described here below. The LNG system 200 of FIG. 3 comprises three closed loops or cycles, labeled 201, 203 and 205 respectively. Three different refrigerants are processed in the three loops. A first refrigerant processed in loop 201 can be propane. The first loop 201 will be named the pre-cooling loop here below. A second refrigerant processed in loop 203 can be ethylene and a third refrigerant circulating in loop 205 can be methane. A natural gas line 207 flows through three sequentially arranged heat exchangers 209, 211 and 213 of the three loops 201, 203, 205. The natural gas enters the first heat exchanger 209 in the gaseous state and exits the last heat exchanger 213 in the liquid state.

The system of FIG. 3 is represented in a somewhat simplified manner. The first, pre-cooling loop or cycle 201 comprises an integrally-geared turbo-compressor 229 including a plurality of compressor stages. In some embodiments, three compressor stages 229A-229C can be provided, as shown by way of example in the schematic representation of FIG. 3. In other embodiments a different number of compressor stages can be provided. In general, the number of compressor stages can depend upon the number of side streams provided in the pre-cooling loop 201, in a way similar to what has been disclosed in connection with FIGS. 2 and 2A.

Inlet guide vanes 228C, 228B, 228A can be provided at the inlet of some, and more particularly of each compressor stage. Intercoolers can be arranged between pairs of sequentially arranged compressor stages, for example a first intercooler 230 can be arranged between the delivery side of the first compressor stage 229C and the suction side of the second compressor stage 229B. A further intercooler 231 can be arranged between the delivery side of compressor stage 229B and the suction side of compressor stage 229A.

The delivery side of the last compressor stage 229A, i.e. the most downstream one in the pressure-increasing flow direction, is connected to a condenser 233. The first refrigerant circulating through the integrally-geared turbo-compressor 229 is condensed in the condenser 233 and delivered through a line 235 to the first heat exchanger 209. The compressed and condensed refrigerant flow can be expanded through one or more expansion elements, one of which is shown at 237. In a way similar to FIG. 2, the main refrigerant stream flowing in delivery line 235 can be divided into side streams at gradually decreasing pressures and temperatures. The heat exchanger 209 can be comprised of a plurality of heat exchanger sections arranged in series and through which a fraction of the refrigerant is caused to flow at gradually decreasing pressures, in a way quite similar to what has been described in connection with FIGS. 2 and 2A. A plurality of side streams are thus formed, each being returned at a respective one of the compressor stages 229A, 229B, 229C.

Each compressor stage processes, therefore, a different refrigerant flow rate at variable and gradually increasing pressures from the most upstream compressor stage 229C through the most downstream compressor stage 229A.

The integrally-geared turbo-compressor 229 can be driven by a prime mover. In some embodiments the prime mover can be an electric motor, not shown, similarly to motor 111 described with reference to FIG. 2. In other embodiments the prime mover can comprise a gas turbine, for example an aeroderivative gas turbine.

The second loop 203 comprises compressor arrangement 241. The compressor arrangement 241 can comprise a single compressor or a plurality of sequentially arranged compressors. One or more of the compressors of the compressor arrangement 241 can be a multi-stage compressor, e.g. a multi-stage centrifugal compressor. The compressor arrangement 241 can be driven by a second prime mover 243. In some embodiments the second prime mover 243 can comprise a gas turbine, for instance an aeroderivative gas turbine. In other embodiments the prime mover can comprise an electric motor. Combinations of different engines or motors can be envisaged as well.

The second loop 203 comprises a condenser 245 through which the compressed second refrigerant delivered by the compressor arrangement 241 is condensed. A delivery line 247 delivers the compressed and condensed second refrigerant through the first heat exchanger 209 and through the second exchanger 211. In the first heat exchanger 209 the condensed second refrigerant is cooled by exchanging heat against the first refrigerant circulating in the first loop 201. In the second heat exchanger 211 the second refrigerant is expanded in one or more sequentially arranged expansion elements, one of which is shown at 249. In a manner known per se, the streams of second refrigerant at different and gradually reducing pressures can thus be generated, the side streams being returned through return lines 251, 253, 255 at decreasing pressures to the second compressor arrangement 241. In some embodiments, each side stream is injected at the inlet of a respective one of a plurality of serially arranged compressors forming the compressor arrangement 241. Movable inlet guide vanes can be provided at the inlet of each such compressors. In the second heat exchanger 211 the second refrigerant cools and/or partly liquefies the natural gas flowing through gas line 207.

The third loop 205 comprises a further compressor arrangement 261. The compressor arrangement 261 can be comprised of a single compressor or a plurality of sequentially arranged compressors. The compressor(s) of the compressor arrangement 261 can be centrifugal compressors, e.g. multi-stage centrifugal compressor. A further prime mover 263 is provided for driving the compressor arrangement 261 into rotation. In some embodiments, the prime mover 263 can comprise a gas turbine, for instance an aeroderivative gas turbine. In other embodiments the prime mover 263 can comprise an electric motor. Combinations of different motors and engines can be provided as well.

The compressed third refrigerant delivered by the compressor arrangement 261 is condensed in a condenser 265 and delivered in the liquid state through a delivery line 267 through the first, the second and the third heat exchangers 209, 211, 213. In the first and second heat exchangers 209, 211 the third refrigerant flows in the liquid state and is cooled by exchanging heat against the first refrigerant and the second refrigerant, respectively. In the last section of the loop, the third refrigerant is expanded in one or more sequentially arranged expansion elements 269. The vaporized third refrigerant exchanges heat against the natural gas in the third heat exchanger 213, until the natural gas is liquefied when delivered from the third heat exchanger 213. In some embodiments, the third refrigerant can be subdivided into side streams at gradually reducing pressures and each side stream is returned to the compressor arrangement 261 through respective return line 271, 273, 275. Also in this case, side streams can be injected at the inlet of sequentially arranged compressors forming part of the compressor arrangement, each compressor being possibly provided with movable inlet guide vanes.

The LNG process described so far and illustrated in FIG. 3 is known as a cascade process. As noted above, differently from known cascade processes and systems, in the present embodiment at least the first, pre-cooling loop 201 comprises a multi-stage integrally-geared turbo-compressor.

Processing the first refrigerant in the pre-cooling loop through the integrally-geared turbo-compressor has several advantages as already described in connection with the embodiment of FIG. 2, 2A. The integrally-geared turbo-compressor 229 of the embodiment of FIG. 3 can be conceptually similar to the integrally-geared turbo-compressor described with respect to the embodiment of FIGS. 2, 2A and shown in more detail in FIG. 4. Merely by way of example the number of compressor stages of the integrally-geared turbo-compressor 229 shown in FIG. 3 is different from the number of stages of the embodiment of FIGS. 2, 2A, this indicating that the number of stages of the integrally-geared turbo-compressor can vary based on design considerations, for example depending upon the number of side streams into which the main stream of the first refrigerant is divided downstream of the condenser.

In some embodiments the integrally-geared turbo-compressor can be driven at a power ranging from about 12 MW to about 40 MW. In some embodiments, the integrally-geared turbo-compressor can have a rated power ranging between about 14 MW and 40 MW and more specifically between about 25 MW and 30MW.

In some embodiments a first refrigerant flow rate ranging from about 10,000 m³/h to about 70,000 m³/h can be processed by the integrally-geared turbo-compressor.

As disclosed above, the first refrigerant in the LNG system is usually expanded at gradually reducing pressure values and divided into side streams, each stream being returned to a respective one of several compressor stages of the integrally-geared turbo-compressor. In some embodiments, the delivery pressure of the most downstream compressor stage, i.e. the compressor stage at the highest pressure, ranges from about 45 bar absolute to about 65 bar absolute and in some embodiments the delivery pressure can range between about 52 bar absolute and about 56 bar absolute. In some embodiments, the respective suction pressure, i.e. the pressure at the inlet of the most upstream compressor stage, can range between about 2.5 and about 15 bar absolute, and more specifically e.g. between about 3 and about 10 bar absolute, for instance at around 3-3.5 bar absolute.

In other embodiments, the delivery pressure (discharge pressure) of the last stage in the integrally-geared turbo-compressor can range between about 10 bar absolute and 30 bar absolute, and in some specific embodiments between 15 and 25 bar absolute. The respective suction pressure at the most upstream compressor stage can range between about 1 and about 2.5 bar absolute, more specifically between about 1.5 and about 2 bar absolute, for instance at around 1.6-1.9 bar absolute.

The use of an integrally-geared turbo-compressor in the precooling cycle results in enhanced efficiency of the compressor and thus reduced power consumption, and finally in considerable cost savings when compared with a current art centrifugal multi-stage compressor.

To fully appreciate the important advantages in terms of increased efficiency and reduced energy consumption and the cost savings achieved thereby, the following comparative example shall be considered.

In a system according to FIG. 1, a standard configuration using a beam centrifugal compressor, for example a 3MCL804 manufactured by GE Oil & Gas, Florence, Italy with an efficiency of “100%”, driven by a PGT25+G4 aeroderivative gas turbine, available from GE Oil & Gas, Florence, Italy, directly coupled to the compressor with the following operating condition:

inlet pressure 1.13 bar absolute

inlet volumetric flow 56,000 m³/h

rotary speed 6,100 rpm

the compressor would absorb 21,108 kW at design condition. An arrangement according to FIGS. 2, 2A, comprising an integrally-geared turbo-compressor, for example an SRL804 manufactured by GE Oil & Gas, Florence, Italy driven by an equivalent gas turbine and having an efficiency of 102.4% would absorb, under the same operative conditions, 20,493 kW, which involves a reduction of 3% of the power consumption.

The total cost saving with the integrally geared configuration is 5%.

The use of an integrally geared compressor is even more attractive considering a configuration wherein an electric motor is used, instead of a gas turbine, due to the removal of the gearbox. In a standard solution according to the prior art using an electric motor as a driver, a fixed speed electric motor is drivingly connected to the compressor through a gearbox. Conversely, if an integrally geared compressor is used, the compressor can be designed at the optimum speed without the additional gearbox. The compressor will reach an efficiency up to 104.1%. Under the above mentioned operating conditions this would result in an absorbed power of 20,102 kW, which results in a reduction of power consumption of 1006 kW. In term of cost a solution with an integrally geared compressor and an electric motor is 14% less expensive than a standard solution with electric motor, gearbox and compressor, mainly thanks to the removal of the gearbox.

While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. 

What is claimed is:
 1. A natural gas liquefaction system comprising: at least a pre-cooling loop, through which a first refrigerant is adapted to circulate, the pre-cooling loop comprising: at least one compressor for pressurizing the first refrigerant; at least one prime mover for driving said compressor; at least one condenser for removing heat from the first refrigerant; at least a first expansion element for expanding the first refrigerant; at least a first heat exchanger for transferring heat from natural gas to the first refrigerant; and at least a cooling loop, downstream of said pre-cooling loop, where through a second refrigerant circulates, the natural gas being adapted to be sequentially cooled in the pre-cooling loop and in the cooling loop; wherein said compressor is an integrally-geared turbo-compressor comprising a plurality of compressor stages each one being provided with independent set of movable inlet guide vanes for autonomously regulating flows entering in the compressor stages.
 2. The system of claim 1, wherein the pre-cooling loop is configured to divide the first refrigerant into two or more side streams returned at respective compressor stages of said integrally-geared turbo-compressor.
 3. The system of claim 1, wherein said prime mover comprises an electric motor.
 4. The system of claim 1, wherein said prime mover comprises a gas turbine.
 5. The system of claim 1, wherein the pre-cooling loop further comprises: a plurality of sequentially arranged first expansion elements configured for expanding the first refrigerant at a plurality of decreasing pressure levels; a plurality of first heat exchangers arranged and configured for receiving respective portions of said first refrigerant expanded through said plurality of sequentially arranged first expansion elements and for transferring heat from the natural gas to the first refrigerant; a plurality of return paths returning side streams of the first refrigerant from each said first heat exchangers to a respective compressor stage of the integrally-geared turbo-compressor.
 6. The system of claim 1, wherein the pre-cooling loop further comprises: at least a first auxiliary expansion element and at least a first auxiliary heat exchanger receiving a portion of said first refrigerant expanded through said first auxiliary expansion element and configured for transferring heat from the second refrigerant circulating in the cooling loop to the first refrigerant circulating in the pre-cooling loop.
 7. The system of claim 1, wherein the pre-cooling loop further comprises: a plurality of sequentially arranged second expansion elements configured for expanding the first refrigerant at a plurality of decreasing pressure levels; a plurality of second heat exchangers arranged and configured for receiving respective portions of said first refrigerant expanded through said first auxiliary expansion elements and for transferring heat from the second refrigerant to the first refrigerant; a plurality of return paths returning the portions of first refrigerant from each said second heat exchangers to a respective compressor stage of the integrally-geared turbo-compressor.
 8. The system of claim 1, wherein said first refrigerant comprises a gas with a molecular weight greater than
 35. 9. The system of claim 1, wherein said second refrigerant is a mixed refrigerant, or ethylene, or methane.
 10. The system of claim 1, wherein said integrally-geared turbo-compressor comprises: a central gear driven into rotation by said prime mover; a plurality of driven shafts, each driven shaft comprising a pinion meshing with the central gear and driven into rotation by said central gear; at least one respective compressor impeller mounted on each shaft.
 11. (canceled)
 12. The system of claim 1, further comprising at least one intercooler between at least two serially arranged compressor stages of said integrally-geared turbo-compressor.
 13. The system of claim 1, wherein the integrally-geared turbo-compressor compresses the first refrigerant so that the pressurized first refrigerant is delivered from the last stage of the integrally-geared turbo-compressor at a pressure ranging from about 45 to about 65 bar absolute. and more particularly between about 50 and about 60 bar absolute; and wherein the pressure of the first refrigerant at the inlet of the first stage of the integrally-geared turbo-compressor ranges from about 2.5 to about 10, and more particularly from about 3 to about 5 bar absolute.
 14. The system of claim 1, wherein the integrally-geared turbo-compressor compresses the first refrigerant so that the pressurized first refrigerant is delivered from the last stage of the integrally-geared turbo-compressor at a pressure ranging from about 10 to about 30 bar absolute, and more particularly between about 15 and about 25 bar absolute; and wherein the pressure of the first refrigerant at the inlet of the first stage of the integrally-geared turbo-compressor ranges from about 1 to about 2.5, and more particularly from about 1.5 to about 2 bar absolute.
 15. The system of claim 1, wherein the integrally-geared turbo-compressor delivers a first refrigerant flow at a flow rate ranging from about 10,000 actual m3/h to about 70,000 actual m3/h.
 16. The system of claim 1, wherein the integrally-geared turbo-compressor absorbs a power ranging from about 12 MW to about 40 MW and more particularly ranging from about 14 MW to about 30 MW.
 17. The system of claim 1, wherein the integrally-geared turbo-compressor comprises at least four stages, each stage comprising at least one impeller.
 18. A method of liquefying natural gas, comprising: providing a pre-cooling loop comprising: an integrally-geared turbo-compressor having a plurality of compressor stages each one being provided with movable inlet guide vanes, at least one condenser, at least one expansion element, and at least one heat exchanger; driving said integrally-geared turbo-compressor with a prime mover; circulating a first refrigerant through the integrally-geared turbo-compressor; condensing the first refrigerant delivered by the integrally-geared turbo-compressor in the condenser; dividing the first refrigerant in a plurality of partial flows; expanding the condensed first refrigerant in the expansion element; circulating the expanded refrigerant through the heat exchanger to remove heat from the natural gas, to pre-cool the natural gas; controlling independently movable inlet guide vanes to regulate the partial flows at the suction side of the compressor stages; providing at least one cooling loop; circulating a second refrigerant in said at least one cooling loop; remove heat from the pre-cooled natural gas by heat exchange against the second refrigerant.
 19. The method of claim 18, wherein said prime mover comprises an electric motor.
 20. (canceled)
 21. The method of claim 18, wherein the movable inlet guide vanes are controlled as a function of flow conditions of the partial flows.
 22. The method of claim 18, further comprising the steps of: providing a plurality of sequentially arranged first expansion elements in said pre-cooling loop; expanding condensed first refrigerant through said first expansion elements at a plurality of decreasing pressure levels; circulating portions of the expanded first refrigerant from each said first expansion elements through a plurality of first heat exchangers, to remove heat from the natural gas; returning through respective return paths said portions of expanded first refrigerant from said first heat exchangers to respective ones of said plurality of compressor stages.
 23. (canceled) 